Michael E. ReedyReedy International CorporationKeyport, New Jersey, US

Abstract

Chemical foaming agents can play a key role which enables both commodity
and engineering polymers to process more easily and with improved properties
for injection molding and extrusion processes. Both additive producers
and resin companies have made improvements to their products. Compatibility
and chemical reactions with blowing agent by-products are becoming more
important considering desired improvements in part weight and impact strength.

Blowing Agent Process

What really happens when you add chemical foaming agents or blowing agents
to your polymer process? The agents are blended with the resin or can
be fed directly into the hopper and from there down through the barrel
to the mold. Heat from the barrel causes a thermal decomposition of the
material and may be either endothermic (heat absorbing) or exothermic
(heat generating). Endothermic foaming agents primarily produce CO2 while
exothermic mostly generate N2.

The liquid CO2 or N2 from the blowing agent is mingled among the liquid
plastic resin molecules and is not typically considered to be miscible
or a homogenous solution by itself. This solubility or miscibility of
the liquid is influenced by the type of resin. Thermoplastic resin is
classified as to its macromolecular structure. An amorphous or unstructured
plastic is irregular with highly branched molecular chains and is transparent.
Semi-crystalline thermoplastics have molecules in orderly or linear chains
and are cloudy or semi transparent. Additives such as flame retardants,
anti-oxidants, pigments, fillers, UV stabilizers, etc. also influence
the solubility properties. The polymer system is influenced by the additives
which effect surface energies or surface tension which can either promote
compatibility and intimate mixing or destabilize the melt promoting separation
and coalescing. However, if the resin and additive package, including
blowing agents, are compatible you will be able to produce a polymeric
emulsion, which is ideal for foam processing. Emulsion principles work
as well for mayonnaise as for new design foamable semi-crystalline resins
from Amoco or Montell.

The basic particle size of the blowing agent determines the size of the
liquid/gas mixture in the melt. Large particles will tend to generate
large liquid/gas mixtures and fine particles will tend to generate small
liquid/gas mixtures. Under the right magnification levels, you can almost
see beach balls vs. golf balls in the matrix.

The size of these liquid droplets and the amount mingled in the polymer
mixture has a lubricating effect. This not only reduces shear-heating
forces but also improves melt flow and consequently the polymer melt index.
Fine particles that produce fine liquid droplets tend to produce fine
emulsions.

Blowing Agent Compatability

Both additive and resin producers continue to make improvements in their
product performance and compatibility. The basic chemical reactions in
the polymer melt are becoming more critical when trying to lower part
weight and maintain or improve impact strength. All chemical blowing agents
decompose thermally to produce gas and chemical by-products. We need to
recognize that these by-products can be either basic, neutral, or acidic
and they can affect the pH balance of the complete polymer system. This
has caused a number of problems with other additives especially flame
retardants, stabilizers, and AZO based colorants.

Many of the low cost / low performance blowing agents in the market have
caused problems with tool corrosion or part embrittlement. For endothermic
agents, too large a concentration of either sodium bicarbonate or citric
acid will cause a reaction with other additives such as bromine compounds
or phosphate esters. This reaction can be with the thermoplastics resin
or the mold tooling. For exothermic agents, the reactions are primarily
basic which help explain why they are the preferred agents for naturally
acidic PVC materials.

The key concept of using blowing agents is to match the agent with the
resin system. When the proper match is achieved, there is a fantastic
processing advantage. For example, a molder with a 15-pound shot of HIPS
was experiencing a 3 _ minute cycle. Using a compatible endothermic blowing
agent, the molder was able to blow out sinks and reduce the cycle to 2
_ minutes—a savings that amounted to almost $1.00 / part. This helps
explain why there are over 150 endothermic formulations and over 200 exothermic
formulations in the market today. Each blowing agent is attempting to
offer the processor a unique economic or performance advantage.

In the future, we anticipate an increasing in endo/exothermics that match
process and performance improvements. Similarly, the resin producers are
interested in expanding their product lines to include more foam friendly
resins and foam friendly additives.

Blowing Agent Use In Injection Molding

There are over 10 separate injection molding processes using blowing
agents. Chemical blowing agents can play a key role and enable both commodity
and engineering polymers to process more easily and with improved properties.
These include:

Straight Injection Molding

Low Pressure Structural Foam Molding

High Pressure Structural Foam Molding

Gas Counter Pressure Structural Foam Molding

Nitrogen Injection Structural Foam Molding

Gas Co-Injection Structural Foam Molding

Gas Assist Molding

Chemical Gas Assist

CoralFoam

Over Molding Structural Foam Molding

Blowing agents in High Pressure Structural Foam Molding create a microcellular
structure with a smooth solid skin around a fine cellular core. Particle
size, distribution purity, and a controlled gas release are tailored to
provide many, very small nucleation sites that create this fine and uniform
microcellular structure.

Gas Counter Pressure Molding uses endo/exothermic agents to eliminate
sink marks and improve processing economics including density reduction
and cycle time with a class “A” finish. With CO2 based blowing
agents, only a 35 p.s.i. counter pressure is needed to prevent splay.

CoralFoam is a new selective foaming process built around sophisticated
tool design and endothermic agents. CoralFoam uses CO2 based blowing agents
because of its low-pressure solubility and predictable post mold foaming
characteristics.

Process Improvement Using CO2

The process benefits are all based in chemistry and the chemical reactions
discussed earlier, as well as the physical nature of the gases. CO2 is
a low vapor pressure gas with low-pressure solubility. It can become a
super critical fluid at relatively low-pressure. Typically, this is in
the range of 1,000 to 1,700 p.s.i. In the super critical state, CO2 is
a super solvent and can lower the TG of most resins. In the case of PVC,
studies have shown reductions of 50∞C. The solubility rate of liquid
CO2.is still being studied. This rate is important in injection molding
processes because the dwell time is very short. What we have typically
been able to observe is that a lowering of the process temperature by
10∞ to 20∞F, overall for most resin processes, is achievable.

In many cases, the endothermic agent is not used to make foams, but will
be used to improve the melt behavior of the polymer as a processing aid.
CO2 acts as a lubricant improving melt flow, which, for injection molding,
improves mold filling. Blowing agents are often used to eliminate sink
marks. The CO2 will seek out area of lowest resistance, which are the
hottest areas. Cellular expansion fills the voids left from the cooling
polymer. One key concept to remember is that you must change the process
parameters to take advantage of foam formation. In most cases, excess
pack pressure will prevent foaming.

Probably the most important advantage of lowering viscosity is the possibility
to achieve equivalent flow characteristics at lower melt temperatures.
By lowering the viscosity of the polymer melt, one has the choice of reducing
the processing temperature or utilizing the improved melt flow at the
same temperature. Improved melt flow can mean fewer gates, thinner wall
sections, less molded-in-stress and reduced burning through shear heat.
In certain cases, the uses of endothermic agents make production of multi-cavity
tools easier to balance.

The saturated CO2 of the emulsion exerts an internal pressure that enables
the processor to feed polymeric material very consistently. Since the
product is still saturated with CO2 and is significantly plasticized,
lower molding stresses are evident in the product. The plastic will orientate
in the direction of the flow (laminar flow) and the blowing agent will
act as an expanding spear causing a molecular disorientation. This results
in far less stress cracking in the finished part and improved impact strength.

Troubleshooting Using Chemical Foaming

One easy way to discuss process improvements using chemical blowing agents
is to highlight the problems and the important variables in resolving
them. Key foam processing troubleshooting actions include:

Foam Post Blow—Because foaming agents are so efficient,
parts removed from the mold will continue to swell. The solution is to
lower the process temperatures, which, as I mentioned earlier, will allow
you to reduce cooling time which, is the largest time component in the
processing cycle. You may also simply be using too much blowing agent
for the part.

Elephant Skin—This is a surface roughness often appearing
near the end of the fill. This is usually an indication that the melt
or mold temperatures are too low.

Sink Marks—As thermoplastic materials cool, they shrink
and thick portions of ribs or bosses shrink the most. Add a little more
blowing agent and make sure that the pack pressure is released so the
foam has room to expand.

Warpage—-This is when the parts side walls bow in or out.
This is often because the temperature and injection pressures are too
high. Also the part is not cooling or setting-up completely, so increased
cooling will help.

Voids—These are areas of missing material, usually hollow
areas formed from gas pockets. The solution is usually to increase melt
temperatures and lower the injection speed. You may want to avoid pressure
drops that will result in inadequate gas counter pressure on the melt
face and cause prefoaming and other anomalies inside the mold. It is very
important to match the foaming agent to the resin system.

Plate Out—Plate out or deposits on the screw and screen
pack is a problem with exothermic blowing agents. This can have a number
of causes including the resin and additive packages, but the quickest
solution is to reduce the amount of blowing agent used. Again, it is important
to ensure that the blowing agent is matched to the resin emulsion system.

Impact and Brittleness—This is a problem when a part does
not meet the physical properties requirement. Typical fixes include lower
temperatures, decrease the amount of blowing agent or change the blowing
agent to a different type. You may want to contact the blowing agent producer
for specific recommendations.

Flash—This is the excess material usually at the mold
knit lines seen after demolding. The solution is to lower the process
temperatures, reduce shot size or lower the blowing agent dosage. Avoid
sharp transitions of section thickness; especially close to the point
of injection because it will create more difficulty with regard to surface
gassing. If possible, smooth transitions should be used.

Burning—This is when the part is discolored and the surfaces
show unevenness. This can be fixed by lowering temperatures and opening
the gates and vents. In general you want to avoid flow constrictions that
may cause excessive shear heat and an undesired reaction of the blowing
agent. Remember gates should be as generous as possible but should still
remain not more than two thirds of the product thickness that is being
fed.

Conclusion

We have briefly discussed how chemical blowing agents can improve performance
and productivity in injection molding processes. Compatibility of the
blowing agent and the complete polymer system is the key consideration.
The effect of CO2 on the flow behavior of a polymer has been well documented
and can be used by the injection molder to produce products with improved
physical properties and process economics.

We are also studying how chemical foaming agent technology utilizes unique
surfactants and high surface area particles that can launch supercritical
CO2 into new areas of material science. These involve using supercritical
CO2 to improve polymer to polymer compatibility through interpenetrating
networks. Also, supercritical CO2 solutions are being evaluated as emulsifiers
for organic and inorganic blowing agents and their contribution to foamed
materials.

Now with increased environmental pressure and a new emphasis on the use
of CO2, I am sure endothermic chemical agents will continue to have tremendous
growth in the plastic industry. New opportunities are available –it
only takes foam.

Acknowledgements

The author would like to acknowledge the following people and companies
for their assistance.
---Mike Caropresso, Consultant
---Larry Currie, Mack Molding Company
---Walt Harfmann, Harfmann Technologies
---Larry Novack, BASF
---Milko Guergov, M & C Advanced Processing